In general, an image forming apparatus can use methods such as switch-back system and one-path system to produce images on both faces of a recording medium such as transfer sheet.
In case of the switch-back system, the recording medium is passed through a transfer unit and then a fixing unit to record an image on one face of the recording medium, and then the recording medium is transported to a sheet inverting route to invert faces of the recording medium. Then, the recording medium is switchbacked to the transfer unit and the fixing unit to record an image on the other face of the recording medium.
In case of the one-path system, a double-side transfer mechanism transfers images to both faces of the recording medium and then the recording medium is passed through a fixing unit. Therefore, images can be recorded on both faces of the recording medium without switchbacking the recording medium.
Accordingly, the one-path system can avoid the following aspects associated to the switch-back system; a cost increase due to providing a switchback mechanism; a longer time period for image forming due to switchback process; a sheet jamming which might occur by switchbacking the recording medium curled by heating of a fixing unit, for example.
However, in the one-path system, a disturbance may likely to occur on images when transporting a recording medium from the double-side transfer mechanism to the fixing unit.
In general, a sheet transporting unit is provided between the double-side transfer mechanism and fixing unit in an image forming apparatus employing the one-path system to transport the recording medium having unfixed images on its both faces.
When such recording medium is transported from the double-side transfer mechanism to the fixing unit in the one-path system, the recording medium may be scratched by components of the sheet transporting unit because one face of the recording medium having an un-fixed toner image contacts such component.
With such scratching, the un-fixed toner image on the recording medium may be disturbed.
If a toner image is transferred only to one face of the recording medium (e.g., transfer sheet), a disturbance of the un-fixed toner image can be prevented by contacting a face of the recording medium having no toner image to the component of the sheet transporting unit. Such a configuration can be designed by modifying a design layout in an image forming apparatus.
However, if toner images are transferred to both faces of the recording medium (e.g., transfer sheet), one face of the recording medium having toner image contacts the component of the sheet transporting unit, thereby a disturbance of the images may occur when transporting the recording medium from the double-side transfer mechanism to the fixing unit.
In one related art, a spur, having a plurality of projections on its peripheral circumference, is provided between a double-side transfer mechanism and a fixing unit in an image forming apparatus, wherein the double-side transfer mechanism can transfer toner images to both faces a recording medium (e.g., transfer sheet), and the spur can be rotated by a driving unit.
With such configuration, the recording medium (e.g., transfer sheet) can be guided (i.e., transported) from the double-side transfer mechanism to the fixing unit in the image forming apparatus.
The spur can support the recording medium by sticking the projections from an underside portion of the recording medium.
With rotation of the spur, the recording medium can be guided (i.e., transported) from the double-side transfer mechanism to the fixing unit while the recording medium is supported by the spur from a underside face of the recording medium.
However, the spur has projections that stick the recording medium, by which un-fixed toner images on the recording medium may be disturbed even if the projections of the spur has a sharp profile.
In another related art device, an image forming apparatus includes a first image forming unit and second image forming unit to produce an image on a recording medium with one-path system.
A first toner image formed in the first image forming unit is transferred to a first intermediate transfer belt serving as image carrying belt, and the first toner image is transferred from the first intermediate transfer belt to a first heatable transfer member.
The first toner image, transferred on the first heatable transfer member, is softened (or melted) by heating the first toner image on the first heatable transfer member before the first toner image is transported to a transfer and fixing nip. Then, the first toner image is transferred and fixed on a first face of the transfer sheet.
Similarly, a second toner image, formed in the second image forming unit, is transferred from a second intermediate transfer belt, serving as image carrying belt, to a second heatable transfer member. The second toner image is similarly softened (or melted) on the second heatable transfer member, and is transferred and fixed on a second face of the transfer sheet.
With such configuration, two images are simultaneously transferred and fixed on each face of the transfer sheet, and thereby the transfer sheet having an unfixed toner image on its both face may not be required to be transported to a fixing unit. Accordingly, an image disturbance caused by a transportation of transfer sheet having unfixed toner image on its face can be prevented.
In such image forming apparatus, a toner image may be transferred from the intermediate transfer belt to the heatable transfer member with an electrostatic transfer method, and a cooling unit is provided to cool the heatable transfer member after the toner image is transferred and fixed on the transfer sheet so that excessive heat may not be conveyed to the intermediate transfer belt from the heatable transfer member.
The electrostatic transfer method may cause dust generation or bleeding of the image with an effect of electric discharge or electric field at an entrance and exit of a transfer nip, by which an image quality may degrade.
Furthermore, the electrostatic transfer method needs a bias voltage charger for conducting electrostatic transfer, by which a manufacturing cost of an image forming apparatus may increase.
Furthermore, if the heatable transfer member is cooled after transferring and fixing a toner image on the transfer sheet, the amount of heat to soften (or melt) the toner image on the heatable transfer member may be unfavorably increased, by which energy consumption in the image forming apparatus may increase.